14 May 2026
µF (microfarad) determines how the motor starts and runs. Voltage determines the maximum your capacitor can safely handle.
Get the µF wrong by more than ±5% and your motor either won’t start, runs hot, or wears prematurely. Get the voltage rating wrong (too low), the cap burns out within weeks. Get the voltage rating wrong (too high), nothing bad happens: you’ve just over-spent slightly.
The capacitor in a motor circuit shapes the phase relationship between the run and start windings. Too low a µF: not enough phase shift, motor stalls or starts slowly. Too high: excess current in the start winding, motor overheats.
Motor run capacitors are typically ±5% tolerance. Motor start capacitors are ±10% (start is a brief, low-duty job). Match the printed value to within that tolerance and you’re golden.
The voltage rating on a capacitor is its maximum. A 450VAC cap is rated to survive up to 450V AC across its terminals. A 370VAC cap is rated to 370V. Your mains is ~230V in NZ, but the cap sees back-EMF spikes from the motor that can hit 1.5-2× rated mains. So 370V is fine for most domestic motor circuits, and 450V gives margin.
You can substitute a higher voltage rating: a 450V cap will work in a 370V slot, same µF. You can’t substitute lower. A 370V cap in a circuit that swings to 450V will fail, often spectacularly.